FIELD OF THE INVENTION
This invention relates to the preparation of substituted or unsubstituted benzyl alcohols by reaction of benzyl halides with alkali metal or alkaline earth metal formates at an elevated temperature in the presence of a benzyl formate to the corresponding benzyl alcohol by contacting the benzyl formate with an alcohol suitably in the presence of an esterification catalyst. This invention is concerned with a commercially feasible process which provides high yields of desired substituted or unsubstituted benzyl alcohols at high space-time yields.
Benzyl alcohols, especially the ones substituted in the benzene ring, are valuable intermediates for organic syntheses, i.e., starting substances for the manufacture of the corresponding benzoic aldehydes. Xylyleneglycols and nuclear substituted xylyleneglycols are, for example, starting materials for the manufacture of polyesters and polyurethanes.
Starting from benzylhalides, such as benzylchlorides or -bromides or xylylenedichlorides, two general methods for the preparation of benzyl alcohols or xylyleneglycols are used:
(a) The direct saponification of benzylhalides or xylylenedichlorides with dilute aqueous alkali metal hydroxide- and -carbonate- solutions, and
(b) a two-step process consisting of a conversion of benzylhalides as well as xylylenedichloride with alkali metal- or alkaline earth acetates to benzyl acetates as well as xylyleneglycolbisacetates which afterwards are saponified to the corresponding benzyl alcohols as well as glycols.
The method of direct saponification mentioned under (a) has the advantage of being a one-step process. On the other hand, considerable quantities of dibenzyl ether occur generally as by-products, as well as polymers of xylylenedichlorides. Therefore, up to 20% of dibenzyl ether may form during alkaline hydrolysis. (Ullmann, volume 4, page 310.)
According to this procedure, benzyl chlorides, i.e., dimethylbenzylchloride or methoxybenzylchloride substituted in the benzene nucleus by one or more methyl groups or another electronegative group, do not produce the desired alcohol as principal product, but the corresponding dibenzyl ether (German Pat. No. P 11 08 677). In this German patent, a two-step process is described.
One procedes in such a way that one heats up benzylhalides or xylyenedihalides, preferably chlorides with alkali metal or alkaline earth acetate in a concentrated aqueous solution until all halogenmethyl groups are changed to acetoxymethyl groups, and, afterwards, benzyl alcohols or xylyleneglycols are freed through saponification with alkali metal or alkaline earth metal hydroxides.
There is no doubt that the formation of dibenzyl ethers or resin-like products is prevented, but the disadvantage of this procedure resides in that it takes long reaction-times for the formation of acetates in the first reaction-step. It is also disadvantageous that high sodium acetate surpluses must be used, amounting up to six-fold the required quantity, restricting the economy very heavily. Besides, a solvent must be used as a reaction-medium, decreasing the utilization capacity of the reactor.
Saponification has been suggested under increased temperatures and pressure (U.S. Pat. No. 2,939,886), with addition of emulsifiers (U.S. Pat. No. 2,819,319) or saponification under heavy shear agitation (DE-AS No. 16 18 530).
Consequently, all of these measures increase the rate of saponification, but do not stop the formation of dibenzyl ether or resin-like by-products. In order to keep the resin content in the reaction product to a minimum, working in dilute solutions is necessary. Therefore, for example, only xylylenedichloride concentrations of a maximum of 5 to 8 weight percent within the aqueous reaction solutions are possible. Therefore, it follows, that after completed saponification heavily dilute xylyleneglycol solutions result which must be concentrated to a gylcol content of 15 weight percent or, in the case of easily water soluble glycols, must be evaporated until complete dryness in order to isolate the glycols by crystallization or extraction. The evaporation of such large amounts of water is economically unsound and constitutes a disadvantage of the procedure.
According to the process of U.S. Pat. No. 2,939,886, the conversion of xylylenedichloride with alkali acetates is carried out in water at increased temperatures and elevated pressure. The separation of xylyleneglycol is achieved through saponification of its bisacetate. This procedure has the disadvantage of an exceptionally long reaction-time of 11 hours until the completion of the first reaction step, and of low glycol-yields after saponification during the second reaction-step. In addition, an autoclave is necessary for the implementation of the conversion.
According to another process (U.S. Pat. No. 3,993,699) in which these disadvantages are said to be eliminated, xylylenedichloride is reacted with an alkali metal or alkaline earth acetate to xylyleneglycolbisacetate in the presence of an aromatic hydrocarbon as adjuvant and a tertiary amine as catalyst. This bisacetate is saponified with aqueous alkaline lye to xylyleneglycol. After separation of the adjuvant and the catalyst, xylyleneglycol is extracted from the aqueous solution.
This particular procedure shows decisive disadvantages also. The conversion to xyleneglycolbisacetate progresses in a satisfactory way only in the presence of an adjuvant. By using large amounts of the adjuvant, valuable utilization space within the reactor is lost. The starting concentration of xylylenedichloride in the reaction mixture amounts to only 25 to 40 weight percent. In addition, through the dilution effect of the adjuvant, the reaction time is considerably lengthened. Another disadvantage resides in that after saponification, only heavily dilute xylyleneglycol solutions are present with a maximum 16 weight percent glycol as well as an additional 20 weight percent sodium acetate as by-product. In order to isolate the xylyleneglycol by means of extraction, a multi-fold amount of the aqueous solution as extraction medium is necessary, and, in order to regain the sodium acetate from the aqueous solution free of glycol, large amounts of water must be evaporated. Therefore, the economical feasability of this process is unsatisfactory.
Furthermore, a process has been described, wherein benzylbenzoate is formed through conversion of benzylchloride and sodiumbenzoate in large quantities in the presence of tertiary amine used as catalyst. Yet, the literature does not mention anything about the preparation of xylyleneglycolbisbenzoate of the corresponding glycol.
As shown in the comparative example, transferring this reaction to xylylenedichloride is impossible, since a reaction mixture consisting of one mol xylylenechloride and 2 mols sodiumbenzoate can not be stirred. The preparation of xylyleneglycol through bisbenzoate would be economically unsound, since only 40 weight percent of the mol mass consists of the xylyleneglycol component.
Within the DE-OS No. 2 731 259 the preparation of substituted benzylformate in o-position through electron-attracting radicals, such as nitro-, cyane- or halogen-, is discussed, but no reference is given to the preparation of benzyl alcohols. The disadvantage of this well known process residues in that the formation of benzylformates derived from benzylhalide and sodium formate requires large amounts of solvents, such as dimethyl sulfoxide, dimethyl formamide or hexamethyl-phosphoric acid triamide and that the benzylformates have to be separated from the solvents through the addition of large amounts of water. In this way, the space- time- yield diminishes with the presence of large amounts of solvents. In order to regain the expensive solvents which are soluble with water in unlimited amounts, large quantities of water have to be evaporated, making the process complicated and economically unsound.
The presented invention is addressed to the problem of supplying an economically improved procedure for the preparation of benzyl alcohols and benzyl alcohols substituted in the benzene ring, for example xylyleneglycols, which can be carried out from a commercial point of view without large structural apparatus and which provides good yields within short reaction times.